The denatured state of a protein is implicated in a number of disease states, and plays a functionally important role in such biologically important process as transport across membranes, protein degradation and protein folding. It is the long-term goal of this work to elucidate the thermodynamic properties of the denatured ensemble, and thereby understand how these properties relate to the compactness of the denatured state, and to the denaturation Gibbs energy changes used in assessing the stability of proteins. This project will focus on a class of proteins whose denatured ensembles continue to change their thermodynamic characters as a function of denaturant concentration. The "variable" thermodynamic behavior of the denatured ensembles of such proteins contrasts dramatically with the "fixed" behavior exhibited by proteins whose denatured ensembles do not change their thermodynamic properties as a function of denaturant. Potentiometric titrations of native and denatured ensembles, as well as proton uptake/release measurements, will be used to: (1) distinguish "variable" from "fixed" thermodynamic behavior of denatured ensembles, and (2) to provide a model-independent means of evaluating the Gibbs energy differences between native and/or denatured states of proteins with related sequences. Size exclusion chromatography will also be used, to correlate dimensional changes in a denatured ensemble with that ensemble's thermodynamic properties, as a function of denaturant concentration. Our ultimate aim is to provide a strong foundation for thermodynamic measurements of protein stability, a foundation that at present is weak or invalid for many proteins.